The present invention relates to the field of optical signal processing.
A number of techniques have been described in the literature that compensate for the effect of multiplicative noise in images. A majority of these techniques are based on reducing temporal coherence or on signal averaging (for example, using moving apertures or rotating diffusers). The drawbacks of these techniques are that reduction of temporal coherence restricts the applicability of coherent optical processors, and that averaging methods are not typically real-time. In addition, digital processing of speckle images may be achieved after electronic detection by averaging, or by applying homomorphic filtering. In this latter technique, the scalar-multiplicative noise (occurring on electronic detection) is logarithmically transformed into an additive noise which can then be reduced by Wiener filtering. One technique for converting multiplicative speckle noise to additive noise entails recording the speckled image in an logarithmic intensity detector, which automatically removes the phase information and provides a logarithmic transform. The fundamental step in all of these image processing techniques is some form of nonlinear intensity conversion at the image plane.
A technique utilized in the present invention is based on optical quadratic processing of the input signal, which immediately results in the output of the square of the magnitude of a complex amplitude input. In the degenerate four wave mixing implementation, this output appears as a scalar amplitude modulation of the phase conjugate of the reference wavefront.